Clear lacquer coat

ABSTRACT

Clear coat layer obtainable by:
         (I) applying an unpigmented intermediate coat to a substrate to be coated;   (II) crosslinking the intermediate coat and forming an intermediate coat layer;   (III) applying an unpigmented top-coat to the inter-mediate coat layer and   (IV) Crosslinking the top-coat and forming a top-coat layer;
 
the intermediate coat layer having a greater flexibility than the top-coat layer, as well as its use in the production of a multilayer coating.

The present invention relates to a clear coat layer, which is suitablein particular for the production of a multilayer coating in theautomotive industry.

In addition to having decorative properties such as imparting color,gloss, etc., coating of motor vehicles fulfills protective functions inparticular with regard to a wide variety of environmental and weatherinfluences, such as acid rain, UV radiation, etc.

However, preventing corrosion of the metal by means of the coating filmis of primary concern, whereby the protective function of the coatingfilm should be guaranteed even under adverse circumstances such as UVradiation, resistance to the impact of stones, mechanical effects(vehicle wash facilities), etc.

These increased requirements have resulted in the use of multilayercoatings in the automotive industry.

The most commonly used multilayer coating is the so-called four-layercoating described below, consisting of four coating layers, each havinga different composition and method of application.

The first layer applied directly to the pretreated automobile sheetmetal is a layer applied electrophoretically (electrocoat layer,cathodic dip coating layer) which is applied by electrodepositioncoating—mainly cathodic dip coating (CDC)—for the purpose of preventingcorrosion and subsequently baked on.

The second layer applied on top of the electrocoat layer andapproximately 20 to 40 μm thick is a so-called primer layer which, onthe one hand, provides protection against mechanical attack (function ofprotecting from impact of stones) while, on the other hand, it smoothsthe rough surface of the body shell for subsequent top coating, fillsminor irregularities and protects the electrophoretically depositedlayer (cathodic dip coating layer) from natural UV radiation. This layeris created largely by applying a baked-on coating, e.g. by electrostatichigh rotation bells and subsequent baking at temperatures above 130° C.

The third layer applied on top of the primer layer is the so-calledbase-coat layer, which imparts the desired color to the automobile bodyby appropriate pigments. The base coat is applied by the conventionalspray method. The layer thickness of this conventional base-coat layeris approximately 12 to 25 μm, depending on the tint. In most cases, thislayer is applied in two process steps, e.g. in a first step byapplication by means of electrostatic high rotation bells followed by asecond application by means of pneumatic atomization. This layer issubsequently subjected to intermediate drying with infrared lamps and/orhot air convection.

The fourth and top-most layer applied on top of the base-coat layer isthe clear coat layer, which is usually applied in one operation by meansof electrostatic high-rotation bells. It imparts the desired gloss tothe automobile body and protects the base-coat from environmentaleffects (UV radiation, salt water, etc.). The layer thickness is usuallybetween 30 and 50 μm.

Subsequently, the base-coat layer and the clear coat layer are bakedjointly at a temperature between 130° C. and 160° C.

Furthermore, it is known from European Patent 568 967 B1 that automobilebody precoated with a color-imparting base-coat layer may be coated withtwo layers of clear coat, the bottom-most clear coat layer of whichbeing heat curable and the top-most clear coat layer being radiationcurable.

An application of this multilayer system to substrates to be shapedsubsequently is not mentioned.

A major disadvantage in the production of this four-layer coating isthat it is equipment-intensive and therefore cost-intensive because ofthe different application methods used.

In addition, the use of coatings for the spray application is no longerappropriate for reasons of environmental policy because considerablequantities of overspray arise during such coating operations.

Moreover, because of the shape of the automobile body, differences intint and different top-coat states may be observed, which cannot beprevented in conjunction with the multilayer system described above.

The automotive industry therefore endeavors to replace the parts of thesheet metal outer of the automobile body which must be coated, e.g. thebonnet, the boot cover, doors, etc. by parts already fully coated in thecolor of the vehicle to minimize the disadvantages described above.

An important prerequisite for this process is the use of so-calledprecoated coils. These coils of metal precoated in the color of thevehicle which can be converted to the desired shape by the automobilemanufacturer by appropriate shaping methods (deep drawing) in the coatedstate. No additional coating is thus is necessary.

From U.S. Pat. No. 5,229,214, for example, it is known to apply twodifferent primer layers onto a galvanized steel coil before shaping, thetop layer having a greater flexibility than the first layer beneath it.

Subsequently, the color-imparting base-coat layer and a clear coat layerare applied to this “double” primer layer.

A major disadvantage of the precoated coils used in the past is thateven before shaping, the coating structure does not conform to theproperties required by the automotive industry with regard to gloss andappearance.

Furthermore, it was impossible to reproduce the tints demanded by theautomotive industry. In particular in the case of effect coatings, thedevelopment of roller structures visible to the naked eye could not beprevented when using precoated coils. Furthermore, the development of aneffect (flop effect) in the mass-produced coating could not be repeated.

These are the main reasons why coils precoated in the color of thevehicle are not being used for mass production of motor vehicles.

The latest developments in the automotive industry are movingincreasingly in the direction of modular design, where the automobilemanufacturer simply fits the modules manufactured by outside companiesto the motor vehicle.

The term “module” should be understood to refer to such parts of themotor vehicle which are prefabricated by a supplier for the automobilemanufacturer and are completely functional when taken alone. Examples ofthis include ready-to-install seats, fully wired dashboards etc.

Because of the available coating technology, it has not been possible sofar to market body parts pre-coated in the color of the vehicle or outershell modules.

The object of the present invention is to provide a clear coat layerwhich is suitable for the production of precoated metal coils from whichparts for motor vehicles can be manufactured by appropriate shapingmethods (deep drawing).

This clear coat layer should be suitable in particular for theproduction of coils pre-coated in the color of the vehicle which conformto the properties required by the automotive industry with respect togloss and appearance.

This object is achieved by a clear coat layer obtainable by:

-   -   (I) applying an unpigmented intermediate coat to a substrate to        be coated;    -   (II) crosslinking the intermediate coat and forming an        intermediate coat layer;    -   (III) applying an unpigmented top-coat to the intermediate coat        layer, and    -   (IV) crosslinking the top-coat and forming a top-coat layer,        the intermediate coat layer having a greater flexibility than        the top-coat layer.

However, the object according to the invention is also achieved by aclear coat layer obtainable by:

-   -   (I) applying an unpigmented intermediate coat to a        color-imparting base-coat layer;    -   (II) crosslinking the intermediate coat and forming an        intermediate coat layer;    -   (III) applying an unpigmented top-coat to the intermediate coat        layer and    -   (IV) crosslinking the top-coat and forming a top-coat layer,        the intermediate coat layer having a greater flexibility than        the top-coat layer.

With the clear coat layer according to the invention, it is possible forthe first time to provide metal coils precoated in the color of thevehicle which can be used for the production of automobile body outershell parts or corresponding modules which satisfy the requirements ofthe automotive industry with regard to appearance and color.

In addition, the clear coat layer according to the invention alsosatisfies the other requirements regarding an automobile series coatingsuch as mechanical resistance to stress.

The clear coat layer according to the invention has two majordifferences in comparison with a “double clear coat layer” in which twolayers have been produced from the same coating. Firstly, the clear coatlayer according to the invention has an improved flexibility, andsecondly, the adhesion between the coat layer and the top-coat layer isso good that it is possible to provide a multilayer coating for metalcoils precoated in the color of the vehicle by way of the systemaccording to the invention. In the case of a “double clear coat layer”the adhesion is so inadequate that it is manifested by the top-mostlayer splintering off after a shaping of a precoated metal coil by deepdrawing.

In addition the appearance of a clear coat layer composed of twodifferent unpigmented coatings each having a layer thickness of 15 μm isbetter than that of a single layer having a thickness of 30 μm.

The term “unpigmented” as used here and below should be understood torefer to such coatings which do not comprise any color-impartingpigments.

Color-imparting pigments include in particular absorption pigmentsand/or fillers, e.g. titanium dioxide, iron oxide pigments, carbonblack, silicon dioxide, azo pigments, phthalocyanine pigments,quinacridone pigments, diketopyrrolopyrrole pigments, pearlescentpigments, indanthrone pigments, talc, mica, kaolin, chalk, bariumsulfate, various silicas, silicates and organic fibers.

The term “unpigmented coating,” however also includes coatings whichcomprise effect pigments.

According to another particularly preferred embodiment, the intermediatecoat layer has a greater flexibility than the top-coat layer. Theflexibility of the intermediate coat layer may be between T0 and T2,that of the top-coat layer between T0.5 and T5 determined according tothe T-bend test. Details regarding the T-bend test are given in theexamples.

The value of the flexibility of the top-coat layer, determined accordingto the T-bend test, should preferably be 0.5 to 4 units, in particularat least two units higher than that of the intermediate coat layer.

According to a preferred embodiment of the present invention, thetop-coat of the clear coat layer is obtainable by polyaddition of anon-aqueous starting mixture comprising:

-   (A) 10 wt. % to 70 wt. % of a non-aqueous solution of a polymer    based on acrylate with an OH number between 100 and 250.-   (B) 10 wt. % to 70 wt. % of a non-aqueous solution of a    fluorine-modified polymer having a glass transition temperature    between 20 and 40° C., and-   (C) 20 wt. % to 60 wt. % of at least one blocked aliphatic or    cycloaliphatic polyisocyanate; the weight ratio of component (A) to    component (B) amounting to at most 1, and the sum of the components    (A), (B), and (C) amounting to 100%, based on the binder content of    the starting mixture to be crosslinked.

This yields an excellent resistance to chemicals and weathering of thefinished clear coat layer, which is better than that of a clear coatproduced only on the basis of fluoropolymer resin according to the stateof the art.

Even better results with respect to gloss are achieved with such a clearcoat layer according to the invention in which component is obtainableby radical polymerization of a monomer mixture comprising the followingcomponents:

-   -   (i) 0 30 wt. % to 60 wt. % of at least one polycycloaliphatic        compound with at least two rings and a refractive index of at        least 1.460 at 20° C.,    -   (ii) 25 wt. % to 70 wt. % of at least one C₂-C₄ hydroxyalkyl        (meth)acrylate with primary hydroxyl groups, and    -   (iii) 0.1 to 1 wt. % acrylic acid, the sum of components (i),        (ii), and (iii) amounting to 100 wt. %, based on the monomer        mixture.

When using this special non-aqueous solution of a polymer based onacrylate, the other properties of a coating film obtained bycrosslinking a corresponding clear coat layer are not impaired; inparticular its mechanical properties (resistance to impact by stones,hardness and flexibility) and its resistance to chemicals satisfy thehigh requirements of the automotive industry with respect to a clearcoat.

A polycycloaliphatic substance comprising a carboxyl group is understoodin the context of the present invention to refer to a substance or acompound which has a polycarboxylic structure or substructure, i.e., therings are only carbocycles. The designation (meth) in (meth)acrylic asused here and below indicates that it includes both the methacryliccompounds and the acrylic compounds.

With regard to the relationship between refractive index and gloss,reference is made to the article by Juergen H. Braun in Journal ofCoatings Technology, Vol. 63, No. 799, August 1991.

With respect to the relationship between the refractive index andtemperature, reference is made to Organikum, Autorenkollektiv [OrganicChemistry, various authors], VEB Deutscher Verlag der Wissenschaften,16th edition, Berlin 1986, p. 76 f.

For substances which are not liquid at 20° C., the refractive index canbe determined at an elevated temperature by using a thermostaticallyregulated Abbé refractometer with the light of the sodium D line λ=589nm. The increment used for correction of temperature is addition: of5·10-4 units per ° C.

Radical polymerization of component (1) is a current method with whichthose skilled in the art are familiar.

The monomer mixture used in the top-coat layer may additionally comprise5 wt. % to 25 wt. % of a vinyl ester of a branched monocarboxylic acidhaving an average of 9 carbon atoms.

Such vinyl esters are conventional commercial products and areavailable, e.g. under the brand name VeoVa9 from Shell.

The use of such vinyl esters is advantageous when high demands are madeof the hardness and resistance to chemicals.

Especially good results are achieved when isobornyl methacrylate is usedas the polycycloaliphatic compound.

The resulting clear coat layers have excellent properties with regard togloss and resistance to chemicals.

Likewise, very good results can be observed when the polycycloaliphaticcompound of component (i) is selected from an acrylic copolymerobtainable by modifying an acrylic copolymer having at least one epoxygroup with a polycycloaliphatic substance having at least two rings andone carboxyl group with a refractive index of at least 1.460 at 20° C.,the epoxy group originating from glycidyl methacrylate.

This special polycycloaliphatic compound of component (i) may be usedalone or in mixture with other polycycloaliphatic compounds to produce anon-aqueous solution of a polymer based on acrylate.

The use of such a polycycloaliphatic compound having a glycidylmethacrylate radical in mixture with isobornyl methacrylate ispreferred.

In another embodiment according to the invention, the molar ratio ofcarboxyl group to epoxy group is between 0.5 and 1.0, preferably between0.8 and 1.0, especially preferably between 0.9 and 1.0. Thepolycycloaliphatic substance comprising a carboxyl group, however, mayalso be a reaction product of at least two compounds; in particularcomponent (i) is one of the above-mentioned polycycloaliphatic compoundswhich has been additionally reacted further at elevated temperature withpolycarboxylic acids and/or their anhydrides to form a half-ester.

In another embodiment according to the invention, the substancecomprising a carboxyl group may have a refractive index of at least1.480 at 20° C.

In this way, further improvements can be achieved with regard to thegloss of the finished clear coat layer without any negative effect onthe other properties.

Especially suitable polycycloaliphatic compounds may includetricycloaliphatic monocarboxylic acids from the group of hydrogenatednatural resin acids, e.g. commercial products such as Foral AX-E fromthe company Hercules BV, adamantane carboxylic acids; and tricyclicmonocarboxylic acids derived from dicyclopentadiene such astricyclodecane derivatives with a carboxyl group (TCD carboxylic acids),in particular tricyclo-[5.2.1.0.^(2.6)]decane-8 carboxylic acid,preferably tetrahydroabietic acid.

In another preferred embodiment of this invention, thepolycycloaliphatic substance comprising a carboxyl group may be areaction product of at least two compounds, at least one of which is apolycycloaliphatic compound having a refractive index of at least 1.460,preferably at least 1.480, at 20° C.

In particular, at least one of the polycycloaliphatic compounds having arefractive index of at least 1.460 or 1.480 at 20° C. may be comprisedin an amount of at least 10 wt. %, preferably at least 20 wt. %, and inparticular at least 50 wt. %, in the reaction product comprising acarboxyl group.

In particular, a tricycloaliphatic monoalcohol from the group ofperhydrogenated natural resins such as perhydroabietyl alcohol; thedicyclopentadiene derivatives e.g.8-hydroxytricyclo[5.2.1.0.^(2.6)]decane,8-hydroxymethyltricyclo[5.2.1.0.^(2.6)]decane,8-hydroxytricyclo[5.2.1.0.^(2.6)]dec-3-ene,9-hydroxytricyclo[5.2.1.0.^(2.6)]dec-3-ene are suitable aspolycycloaliphatic compound.

This monoalcohol reacts with a compound comprising a carboxyl group toform a half-ester during the production of the polycycloaliphaticsubstance comprising a carboxyl group.

Suitable compounds comprising a carboxyl group for this purpose are, inparticular, dicarboxylic acids or their anhydride(s), for example fromthe group of succinic acid (anhydride), glutaric acid (anhydride),quinoline dicarboxylic acid (anhydride), furan dicarboxylic acid(anhydride), pyridine dicarboxylic acid (anhydride), phthalic acid(anhydride), hexahydrophthalic acid (anhydride), tetrahydrophthalic acid(anhydride), methyl hexahydrophthalic acid (anhydride), naphthalenedicarboxylic acid (anhydride) and maleic acid (anhydride).

The term “(anhydride)” as used here and below indicates that both thefree acid and its anhydride are meant.

If the polycycloaliphatic compound to be used as the starting materialfor the reaction product is a polycycloaliphatic dicarboxylic acid orpossible anhydride(s) thereof, such as e.g. those from the group ofhydrogenated natural resin acids, adamantane carboxylic acids andtricyclic monocarboxylic acids derived from dicyclopentadiene, e.g.tricyclo[5.2.1.0.^(2.6)]decane-8 carboxylic acid, preferablytetrahydroabietic acid, then the alcohol may also be an aliphaticmonohydric alcohol, e.g. methanol, ethanol, n-propanol, isopropanol,methoxypropanol, n-butanol, isobutanol, 2-ethyl-1-hexanol, 1-hexanol, aheptyl alcohol, a nonyl alcohol; a fatty alcohol, e.g. octanol, decanol,dodecanol, a glycol monoether, e.g. methyl glycol, ethyl glycol, butylglycol, polyglycol monoether; an aromatic monohydric alcohol, e.g.benzyl alcohol; or a cycloaliphatic monohydric alcohol, e.g.cyclohexanol, cyclododecanol and/or cyclopentanol.

Here again, the reaction product is a half-ester, which is subsequentlypolymerized with the epoxy group originating from the glycidylmethacrylate.

However, the polycycloaliphatic substance comprising a carboxyl groupmay also comprise one or more aromatic compounds. This option will beselected when the gloss of the finished clear coat layer is to beincreased even further.

Such an aromatic compound may preferably originate from the group ofaromatic monocarboxylic acids such as naphthoic acid,benzenemonocarboxylic acids such as benzoic acid, o-toluic acid,m-toluic acid, p-toluic acid, hydroxybenzoic acid, tert-butylbenzoicacid, aromatic heterocyclic monocarboxylic acids such as pyridinecarboxylic acids and furan carboxylic acids.

According to an especially preferred embodiment, the C₂-C₄-hydroxyalkylacrylate or C₂-C₄ hydroxyalkyl methacrylate is selected from2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate.

Very good results are achieved with 2-hydroxyethyl methacrylate and4-hydroxybutyl acrylate.

However, the invention is not limited to the use of C₂-C₄ hydroxyalkyl(meth)acrylates with primary hydroxyl groups. It is also possible to useC₂-C₄ hydroxyalkyl (meth)acrylates in which up to 50% of the primaryhydroxyl groups are replaced by secondary hydroxyl groups.

Examples of C₂-C₄ hydroxyalkyl (meth)acrylates with secondary hydroxylgroups are 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylateand hexanediol-1,6-mono(meth)acrylate.

All compounds which are in solution and not in the form of a dispersionunder the reaction conditions are suitable for use as thefluorine-modified polymer. Examples of especially suitablefluorine-modified polymers are those based on fluorine-comprising vinylether with a fluorine content between 25% and 30%, a glass transitiontemperature between 16° C. and 45° C. and a hydroxyl value between 45and 90. Such polymers are available commercially and are distributed,e.g. under the brand name “Lumiflon®” by the company Zeneca Resins.

The polycycloaliphatic substance comprising a carboxyl group may alsoadditionally comprise one or more aromatic compounds, preferably fromthe group of aromatic polycarboxylic acids such as naphthoic acid,benzenemonocarboxylic acids such as benzoic acid, o-toluic acid,m-toluic acid, p-toluic acid, hydroxybenzoic acid, tert-butylbenzoicacid, aromatic heterocyclic monocarboxylic acids such as pyridinecarboxylic acids and furan carboxylic acids.

If high demands are made regarding the resistance to weathering of theclear coat layer, no aromatic or heterocyclic monocarboxylic acids areused concurrently or the total amount of aromatic rings including vinylaromatics, e.g. styrene, amounts to no more than 30 wt. %, based on thecoating composition.

Component (i) can be obtained by reacting the starting compounds at anelevated temperature, e.g. 60° C. to 200° C., preferably 120° C. to 170°C. The reaction may be performed in the melt or in the presence oforganic solvents such as those conventionally used in the production ofpaint or synthetic resins for paint, e.g. alcohols such as methoxypropanol, butanol, aromatic hydrocarbons, e.g. xylene, petroleumdistillates based on alkylbenzenes, esters, e.g. butyl acetate,methoxypropyl acetate, ketones, e.g. butanone, methyl isobutyl ketoneand mixtures thereof. If necessary, the conventional catalysts forcatalyzing the epoxy/carboxy reaction may be used for this, e.g. alkalimetal hydroxides, e.g. lithium hydroxide monohydrate, tertiary amines,e.g. triethylamine, N,N-benzylmethylamine, triethylbenzylammoniumchloride, benzyltrimethylammonium chloride, as well as mixtures ofdifferent catalysts, usually in an amount of 0.1 to 2 wt. %, based onthe total amount of the components. If the reactions are performed at anelevated temperature, e.g. 150° C. to 170° C., it is generally possibleto omit catalysts. The modifying agents claimed may be added to theacrylic copolymer comprising epoxy groups before the reactiontemperature or they may be added at the reaction temperature in portionsgradually or continuously, taking into account the exothermic reactionalso in the form of solutions, e.g. in organic solvents if they aresoluble in the solvent or form a stable dispersion.

The amount of the polycycloaliphatic substance comprising a carboxylgroup is selected, as mentioned above, so that the ratio of epoxy groupsto carboxyl groups is usually 1:0.5 to 1:1 and depends mainly on theintended application and/or the use of the coating composition.

The reaction is generally terminated as soon as the acid number hasdropped below 20, preferably amounts to 0 to 10. However, acryliccopolymers having a higher acid number, e.g. 25 to 50 may also beproduced. The number-average molecular weight of component (i) may varywithin wide limits and is preferably between 500 and 10,000, especiallypreferably between 700 and 5000, in particular 750 and 2000 (g/mole).

The acid number is between 0 and 50, preferably between 5 and 25 (mgKOH/g resin).

In principle, all blocked polyisocyanates can be used as crosslinkingagents in which the isocyanate groups have been reacted with a compoundso that the blocked polyisocyanate formed is stable with respect to thehydroxyl groups of the polymer at room temperature but will react at anelevated temperature, usually in the range of approximately 90 to 300°C. Any organic polyisocyanates suitable for crosslinking can be used forthe production of the blocked polyisocyanates. Isocyanates comprisingapprox. 3 to approx. 36 carbon atoms, in particular approximately 8 to15 carbon atoms, are preferred. Examples of suitable diisocyanates arethe diisocyanates listed above.

Polyisocyanates with a higher isocyanate functionality may also be used.Examples include tris-(4-isocyanatophenyl)methane,1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotoluene,1,3,5-tris-(6-isocyanatohexyl)biuret,bis-(2,5-diisocyanato-4-methylphenyl)methane and polymericpolyisocyanates such as dimers and trimers of diisocyanatotoluene.Furthermore, mixtures of polyisocyanates may also be used.

The organic polyisocyanates that may be used as crosslinking agents inthis invention may also be prepolymers which are derived from a polyol,for example, including a polyether polyol or a polyester polyol. To doso, it is known that polyols are reacted with an excess ofpolyisocyanates, thus forming prepolymers with terminal isocyanategroups. Examples of polyols that may be used for this purpose are simplepolyols, e.g. glycols such as ethylene glycol and propylene glycol andother polyols such as glycerol, trimethylolpropane, hexanetriol andpentaerythritol; also monoethers such as diethylene glycol anddipropylene glycol as well as polyethers which are adducts of suchpolyols and alkylene oxides. Examples of alkylene oxides suitable forpolyaddition onto these polyols to form polyethers include ethyleneoxide, propylene oxide, butylene oxide and styrene oxide. Thesepolyaddition products are generally referred to as polyethers withterminal hydroxyl groups. They may be linear or branched. Examples ofsuch polyethers are polyoxyethylene glycol with a molecular weight of1540, polyoxyproylene glycol with a molecular weight of 1025,polyoxytetramethylene glycol, polyoxyhexamethylene glycol,polyoxynonamethylene glycol, polyoxydecamethylene glycol,polyoxydodecamethylene glycol and mixtures thereof. Other types ofpolyoxyalkylene glycol ethers may also be used. Particularly suitablepolyether polyols are those obtained by reacting such polyols asethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol,1,3-butanediol, 1,6-hexanediol and mixtures thereof; glycerol,trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol,dipentaerythritol, tripentaerythritol, polypentaerythritol, methylglycosides and sucrose with alkylene oxides such as ethylene oxide,propylene oxide or mixtures thereof.

Any suitable aliphatic, cycloaliphatic or aromatic alkyl monoalcoholsmay be used for blocking the polyisocyanates. Examples in this respectare aliphatic alcohols such as methyl alcohol, ethyl alcohol,chloroethyl alcohol, propyl alcohol, butyl alcohol, amyl alcohol, hexylalcohol, heptyl alcohol, octyl alcohol, nonyl alcohol,3,3,5-trimethylhexyl alcohol, decyl alcohol and lauryl alcohol; aromaticalkyl alcohols such as phenylcarbinol and methyl phenylcarbinol. Smallamounts of higher molecular weight monoalcohols having a relatively lowvolatility may also be used if necessary, these alcohols acting asplasticizers in the coatings after they are split off.

Other suitable blocking agents are oxime such as methyl ethyl ketoneoxime, acetone oxime and cyclohexanone oxime as well as caprolactams,phenols and hydroxamic acid esters. Preferred blocking agents includemalonic ester, acetoacetate ester and β-diketones.

Methyl ethyl ketoxime and caprolactam are especially preferred.

The blocked polyisocyanates are produced by reacting the capping agentin a sufficient quantity with the organic polyisocyanate so that thereare no longer any free isocyanate groups present.

The blocked aliphatic or cycloaliphatic polyisocyanate is preferably ablocked isophorone diisocyanate (IPDI,3,5,5-trimethyl-1-isocyanato-3-isocyanatomethyl-cyclohexane) present intrimerized or biuret form and/or2,4,6-trioxo-1,3,5-tris(6-isocyanatohexyl)hexahydro-1,3,5-triazine(Desmodur® N3300).

However, other suitable polyisocyanates may also be used, e.g.1,3-bis(1-isocyanato-1-methylethyl)benzene (TMXDI, m-tetramethylxylylenediisocyanate) or 4,4′-dicyclohexylmethane diisocyanate (Desmodur® W).

The latter polyisocyanates must also be reacted with suitable blockingagents.

The choice of suitable blocking agent depends on the crosslinkingtemperatures. When using the clear coat layer according to the inventionas a coil coating paint, methyl ethyl ketoxime or caprolactam is usuallyselected as the blocking agent.

However, there are also commercial blocked polyisocyanates such as forexample those brought on the market by Bayer under the trade nameDesmodur® BL 3175.

According to another particularly preferred embodiment of the presentinvention, the intermediate coat is obtainable by crosslinking anon-aqueous starting mixture comprising in turn:

-   (A) 60 to 90 wt. % of a non-aqueous solution of at least one    cycloaliphatic polyester with an OH number between 20 and 150 and a    glass transition temperature between 0 and 70° C. and a    weight-average molecular weight between 750 and 7000; and    -   (B) 10 to 40 wt. % of at least one blocked aliphatic or        cycloaliphatic polyisocyanate; the sum of components (A) and (B)        being 100%, based on the binder content of the starting mixture.

Those polyesters which are obtainable on the basis of cycloaliphaticsare particularly suitable for the production of intermediate coatsresistant to weathering.

The thickness of the intermediate coat layer may be 10 to 25 μm in thecrosslinked state, the thickness of the top-coat layer in thecrosslinked state may be 10 to 25 μm.

In another embodiment which is also particularly preferred, theintermediate coat layer additionally comprises effect pigments,especially aluminum particles.

The term “effect pigment” is understood to include lamellar pigmentsusually used in effect coatings such as metal pigments, e.g. those oftitanium, aluminum or copper; interference pigments such as metaloxide-coated metal pigments, e.g. aluminum coated with titanium dioxideor mixed oxides, coated mica, e.g. mica coated with titanium dioxide ormixed oxides, microtitanium dioxide and graphite effect pigments,lamellar iron oxide (micaceous iron oxide), molybdenum sulfide pigments,lamellar copper phthalocyanine pigments and bismuth oxychloride flakes,coated glass flakes.

The effect pigments are usually incorporated into the base-coat layer toproduce a multilayer coating for the automotive industry. The problemhere is that the color-imparting pigments cover the effect pigments andthus reduce their effect. In particular in the case of the metallicblack tint, this effect causes a large proportion of the effect pigmenthaving to be added to the base-coat. In addition, the visuallyperceivable effect is much greater if the effect pigments are present inthe intermediate coat layer. It is thus possible for the first time toprovide new possibilities in the use of effect pigments, in particularin the field of coloristics.

The intermediate coats and top-coats required for producing the clearcoat layer according to the invention additionally comprise, besides theobligatory components, the solvents generally used in solutionpolymerization of acrylic copolymers and in the production of baked-oncoatings; the solvents include aromatic hydrocarbons, e.g. xylene,esters, e.g. methoxypropyl acetate, ketones, e.g. butanone, methylisobutyl ketone, alcohols, e.g. butanol, methoxypropanol, glycolmonoether, e.g. butyl glycol and mixtures thereof, e.g. mixtures ofprimarily aromatic petroleum distillate solvents having a higher boilingpoint and butanol and they can be diluted with these solvents or solventmixtures to the application viscosity.

The intermediate coat and/or the top-coat may optionally also comprisethe usual additives and auxiliary substances for production of coatingssuch as:

-   -   surfactants, e.g. wetting agents and flow control agents based        on silicone, e.g. polyether-modified dimethylpolysiloxane        copolymers, fluorosurfactants;    -   rheological aids, e.g. anti-sagging agents (SCA-modified acrylic        copolymers; SCA=sagging control agents);    -   thickeners or thixotropy agents, highly-dispersed silica,        polyurethanes, high-viscosity acrylic copolymers with acrylic        acid and/or methacrylic acid as the main effective        copolymerising component; acid catalysts, e.g. phosphoric acid,        acid half-esters of phosphoric acid with monohydric or dihydric        alcohols, e.g. phosphoric acid monobutyl ester, half-esters of        dicarboxylic cids and/or the anhydrides thereof with monohydric        alcohols, e.g. maleic acid onobutyl ester, solutions of        polyacids in suitable organic solvents, e.g. 20% olutions of        maleic acid in methoxypropanol;    -   accelerators, e.g. tertiary amines, e.g. triethylamine,        dibutyltin dioxide, ibutyltin dilaurate, metal alcoholates, e.g.        aluminum isopropylate, butyl itanate, metal chelates of        aluminum, zirconium or titanium, e.g. titanyl cetylacetonate;    -   light stabilizers, e.g. benzotriazole derivatives and HALS        compounds (HALS=hindered amine light stabilizer);    -   additional crosslinking agents, in particular        -   carboxy-functional components, preferably polycarboxylic            acids or the anhydrides thereof, e.g. itaconic acid,            citraconic anhydride, dodecanedioc acid, 2-dodecenedioc            acid, dodecenylsuccinic anhydride, phthalic anhydride,            tetrahydrophthalic anhydride, trimellitic anhydride, 1,2-,            1,3- and 1,4-cyclohexane dicarboxylic acid,            hexahydrophthalic anhydride and/or mixtures thereof such as            those generally used to harden polyepoxides, e.g. diepoxides            based on bisphenol A, cycloaliphatic diepoxides, e.g.            hexahydrophthalic acid diglycidyl ester,            3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane            carboxylate, acrylic copolymers comprising epoxide groups,            with more than one, preferably two or more epoxide groups            per average molecular weight, or polyacids which are            half-esters obtained by reacting a polyol, e.g.            1,6-hexanediol, trimethylolpropane, with an acid anhydride,            e.g. hexahydrophthalic anhydride, methyl hexahydrophthalic            anhydride, such as those described in European Patent A 212            457, and carboxy-functional acrylic copolymers, e.g. those            synthesized by using substantial quantities of (meth)acrylic            acid in the synthesis of acrylic copolymers, e.g. with an            acid number of 70 or higher as well anhydride acrylic            copolymers, e.g. those produced by using maleic anhydride            and/or itaconic acid in the production of acrylic            copolymers, as described in European Patents A 358 306, A            316 873, as well as unsaturated or saturated polyesters            comprising carboxyl groups, in particular those with a high            acid number, e.g. 70 or more and/or        -   aminoplastic substances which have satisfactory            compatibility in terms of coating technology with the            modified acrylic copolymers, aminoplastics etherified            preferably entirely or partially with monohydric alcohols,            in particular C1-C4 alcohols, e.g. urea and/or            triazine-formaldehyde resins, in particular            melamine-formaldehyde resins, benzoguanamine resins, e.g.            tetramethoxybenzoguanamine, triazine-formaldehyde resins            produced according to unexamined German Patent 42 37 515,            e.g. by reacting 2,4-diamino-6-diethylaminotriazine,            paraformaldehyde and butanol, hexamethoxymethylmelamine,            hexamethylbutoxymethylmelamine,            tetramethoxymethylglycoluril; in particular derivatives            comprising carboxyl groups and derived from partially or            entirely etherified aminoplastics such as those described in            Unexamined German Patent 35 37 855, U.S. Pat. No. 3,519,627,            U.S. Pat. No. 3,502,557 and U.S. Pat. No. 4,026,855; or the            aminoplastics mentioned in the relevant literature (Karsten,            Enamel Raw Material Tables, 9th edition, Curt R. Vincentz            Verlag, Hannover, 1992, pp. 269-288; European Resin            Directory 1983, European Resin Manufacturers Association,            pp. 101-108); and/or        -   TACT (tris(alkoxycarbonylamino)-1,3,5-triazine) such as            tris(methoxycarbonylamino)-1,3,5-triazine,            tris(butoxycarbonylamino)-1,3,5-triazine or mixtures            thereof.    -   other binder components, preferably resins which are        satisfactorily compatible with the intermediate coat and/or the        top-coat from the standpoint of coating technology, in        particular acrylic copolymers comprising carboxyl groups and        hydroxyl groups and/or saturated or unsaturated polyesters        comprising carboxyl groups and hydroxyl groups in subordinate        amounts (1 to 30 wt. %), based on the solid binder.

The solids content of this coating composition in a ready-to-use formpreferably amounts to at least 45 wt. %, in particular 50 wt. % or more.

The clear coat layer may be used in particular for coating precoatedmetal coils (coil coating). The intermediate coat layer and the top-coatlayer are applied by using coating methods with which those skilled inthe art are familiar. Particularly suitable methods include the rollingmethod and the casting head method.

With regard to the use of the clear coat layer for the production ofprecoated metal coils, this is used in multilayer coating. Such amultilayer coating suitable for the automotive industry is obtainablee.g. by the following procedure:

-   -   applying a primer layer to a pretreated metallic substrate and        baking the same at a temperature between 180 and 260° C.;    -   applying a color-imparting base-coat layer and baking the same        at a temperature between 180 and 260° C.;    -   producing a clear coat layer according to the invention (baking        temperature of the intermediate coat and top-coat between 180        and 260° C.).

The coating agents that can be used to produce a primer layer and abase-coat layer in the aforementioned multilayer coating are availablecommercially, e.g. under the trademark Polycoat® CC-Primer from thecompany Bollig & Kemper.

The layer thicknesses of such as a multilayer coating, i.e., thethickness of the primer layer, the base-coat layer and the intermediatecoat layer and top-coat layer in the crosslinked state are between 10and 25 μm respectively.

Very uniform layer thicknesses are obtained by applying the individuallayers by the so-called coil coating method. This yields a particularlyuniform observable effect when using base-coats comprising effectpigments; such an effect could not be achieved in the past by sprayapplication of a base-coat. The minor unavoidable differences in layerthickness which occur with a spray application are manifested in adifference in effect which is clearly observable visually.

The clear coat layer in the present case is especially preferablysuitable for the production of automobile parts by deep drawing themetal coils precoated in the color of the vehicle since the clear coatlayer according to the invention is characterized in particular byexcellent deep drawing properties without any negative effect on theoverall level of properties of the crosslinked coating.

The following examples are presented to illustrate this invention.

EXAMPLES

Production of an Intermediate Coat

In a 2-liter four-necked, round bottomed flask equipped with a heater,thermometers, agitator, column and water separator, a mixture of:

-   -   303.1 g hexahydrophthalic anhydride    -   254.6 g neopentyl glycol    -   8.1 g trimethylolpropane    -   17.6 g maleic anhydride    -   35.2 g adipic acid    -   0.5 g dibutyltin oxide        is melted and heated gradually to 240° C. while constantly        passing nitrogen through it. The water formed as a byproduct        during the polycondensation process is removed continuously.

After reaching an acid number of less than 10 mg KOH/g and a viscosityof 55 to 65 seconds, measured according to DIN 53211 in a 4 mm beaker at20° C. (50% in Solvesso 150), the mixture is cooled and diluted to asolids content of 60% at 120° C. using a mixture of 75.18 g dibasicester, 152.86 g Solvesso 200 and 152.86 g methoxypropyl acetate.

Production of a Cycloaliphatic Acrylate Polymer

Into a 2-liter four-necked, round bottomed flask equipped with a heatingdevice, thermometer, agitator, cooling attachment and gas inlet tube,328.5 g Solvesso® 100, 87.6 g Veova® 9 and 5.8 g cumene hydroperoxide(80% delivery form in a ketone mixture) are placed as startingmaterials. While stirring and passing nitrogen through the mixture, itis heated to 140° C., and using a dripping funnel, a mixture of 284.5 gisobornyl methacrylate, 206.3 g 2-hydroxymethyl methacrylate, 2.3 gacrylic acid, 43 g ethyl-3,3-di(tert-amylperoxy)butyrate and 14.0 gSolvesso® 100 is metered in uniformly within four hours. One hour afterthe end of this addition, a mixture of 4 gethyl-3,3-di(tert-amylperoxy)butyrate and 24.0 g Solvesso® 100 is addeddropwise within 30 minutes. After another two hours, the mixture iscooled to 80° C. and filtered through a 30 μm screen. The resultingresin has an acid number of 4 mg KOH/g, a solids content of 60% and aviscosity of 40 to 60 seconds, measured according to DIN EN ISO 2431 ina 4 mm beaker at 20° C. (50% in Solvesso® 100).

Production of a Top-Coat

In a 2-liter metal mixing vessel, the following are mixed: 360 g of theacrylic copolymer described above are mixed with 140 g of a commercialfluoropolymer resin (Lumiflon® LF 552 from Zeneca Resins, a 60% solutionin aromatic solvents).

Subsequently, 150 g of a commercial blocked aliphatic polyisocyanate(Desmodur® BL 3175 from Bayer AG), 175 g of a commercial blockedcycloaliphatic polyisocyanate (Vestanat® B 1370 from Degussa Hüls AG),20 g of a UV absorber based on benzotriazole (Tinuvin® 1130 from CibaSpezialitäten Chemie), 10 g of a HALS compound (Tinuvin® 292 from CibaSpezialitäten Chemie), 3 g of a flow control agent based on acryliccopolymer (Disparlon® L1984 from Kusumoto Chemicals), 2 g dibutyltindilaurate and 40 g butyl diglycol acetate are added.

After adding 10.0 parts by weight Solvesso® 150, the processingviscosity is adjusted to 80 seconds in the 4 mm DIN beaker at 20° C.

Production of a Base-Coat

560 g of a commercial polyester resin (Dynapol® LH830 from Degussa HülsAG, 60 % dissolved in Solvesso® 150) is placed in a 2-liter metal mixingvessel. With the help of a suitable dispersing device (dissolver fromPendraulik), 5 g colloidal silica (Aerosil® R972 from Degussa AG) isdispersed. Adding 90 g of a commercial blocked aliphatic polyisocyanate(Desmodur® BL 3175 from Bayer AG) is done while stirring the addition of5 g of a flow control agent based on acrylic copolymer (Disparlon® L1984from Kusumoto Chemicals) and 2 g dibutyltin dilaurate (reactionaccelerator) and 50 g Solvesso® 200S.

Then 90 g aluminum effect pigment (Alpate® 8160 from Alcan Toyo) isprepared to a paste in 100 g Solvesso®) 150 and added to the mixturedescribed above after one hour.

The viscosity is adjusted to a value between 90 and 100 seconds(measured in the 4 mm DIN beaker at 20° C.) with approximately 9.8 partsby weight Solvesso® 150.

PRODUCTION OF THE TEST PANELS Example According to the Invention

Chromated aluminum sheet metal conventionally used in the coil coatingindustry, having a sheet metal thickness of 0.58 mm and coated with acommercial anti-corrosion primer suitable for deep drawing Polycoat®21-209-9544 CC-Primer from Bollig & Kemper) with a film thickness of 15μm, is used as substrate for the application of the clear coat layeraccording to the invention.

The base-coat prepared previously is applied to this primer layer so asto yield a dry layer thickness of 15 μm. The base-coat layer is dried ata PMT (peak metal temperature) of 249° C. Subsequently, the intermediatecoat described above is applied to this base-coat layer so that in thecrosslinked state an intermediate coat layer with a dry layer thicknessof 15 μm is obtained. This intermediate coat is also hardened at a PMTof 249° C.

Last of all, the top-coat described above is applied to thisintermediate coat layer so that, in the crosslinked state, a top-coatlayer having a dry layer thickness of 15 μm is obtained. This top-coatis also hardened at a PMT of 249° C.

The resulting multilayer coatings are tested for the followingproperties according to the test methods described below: adhesion,gloss, pencil hardness, cracking after bending, resistance to chemicals,waviness and Knoop hardness.

Table 1 shows the results of these tests.

Comparative Example (Three-Layer)

A multilayer coating operation corresponding to the example according tothe invention is performed except that no intermediate coat layer isapplied. In other words, only a top-coat layer with a layer thickness of15 μm is applied to the base-coat layer.

This multilayer coating is tested for its properties like the multilayercoating according to the invention.

Table 1 shows the results of these tests.

Comparative Example (Four-Layer)

A multilayer coating operation corresponding to the example according tothe invention is performed, except that the intermediate coat layer isproduced by applying and crosslinking the top-coat described above. Inother words, two identical clear coat layers of the same top-coat areapplied to the base-coat layer with the same thickness (15 μm each).

This multilayer coating is investigated for its properties like themultilayer coating according to the invention.

Table 1 shows the results of these tests.

Testing for Adhesion after Swaging

The adhesion or intermediate adhesion was tested by using the methoddescribed in section T6 of the ECCA Testing Standard.

The coat layers applied to an aluminum substrate were provided with agrid cut (in accordance with DIN EN ISO 2409) and then were swaged by 8mm with a swaging device which corresponds to the ISO Standard 1520-1973at the rate stipulated for that test in that standard.

After shaping, the adhesion was determined by using an adhesive coil(Tesa 4104 transparent packing film). To do so, the adhesive film isapplied to the shaped grid cut and then removed at high speed.

Testing the Gloss:

The gloss was determined according to the standards DIN 67539, ISO 2813and ASTM D-523 using a gloss meter from the company Byk-Gardner at ameasurement angle of 20°.

Testing the Pencil Hardness:

The coating surface is scratched at an angle of 45° with the help ofpencils of increasing hardness. The hardness corresponds to that of thehardest pencil which will no longer penetrate into the surface of thecoating. A set of pencils with the following degrees of hardness isused:

-   -   6B-5B-4B-3B-2B-B-HB-F-H-2H-3H-4H-5H-6H.

This test is usually performed by hand, but a mechanical device may alsobe used in which a force of 7.5 Newtons should be applied to the pencil.

This test is described in detail under ECCA-T4 in the ECCA test methods.

The reference standards for this are: ISO 3270-1984/ASTM D-3363-1974(reapproved 1980).

Testing for Cracking after Bending (T-Bend Test):

The coated sheet metal panel is bent 3600 so that the coating film isfacing outwards. Then it is clamped in a vice and pressed togethertightly at the bending point (=>T0).

Using a magnifying glass with a tenfold magnification, the bend isexamined for cracks. If cracks can be detected, the panel is bent arounditself, so that the radius of the bend is increased by the thickness ofthe panel (=>T1).

This procedure is repeated until no more damage occurs. With eachbending operation, the T value is increased by 0.5. The T value at whichno more cracks can be detected is indicated.

This test is described in detail under ECCA-T7 in the ECCA test methods.

The reference standards for this are: EN2370:1991/EN ISO 1519:1995/ENISO 6860:1995/ASTM D 5220-93a.

Testing of the Resistance to Chemicals of Coating Surfaces by Using aGradient Oven:

A gradient oven developed by the company Byk-Mallinckrodt is used toheat, by means of a microprocessor-supported control, a single testsheet which has been coated with the multilayer coating to be tested insuch a way that, after the end of the baking operation, a continuousrange of selectable temperatures for physical tests is available.

Within a working range of +50° C. to +250° C., up to four differentheating zones with a constant temperature may be set as desired.

To test for resistance to chemicals, the following procedure is used:

-   -   The gradient sheet metal is coated with the paint to be tested        and baked.    -   The test chemicals (in line with the client specification) are        applied to the coating film in rows of equal distance. Up to        five chemicals may be applied to the sheet metal at the same        time.    -   The test sheet is placed in the gradient oven, which has been        preheated (client specifications) and this is closed.    -   After a treatment time of 30 minutes (client specification), the        oven is opened and the temperature zones are printed out.    -   The sheet metal removed from the oven is cleaned under running        water (client specification) and then evaluated.    -   The evaluation is performed once immediately and/or after 24        hours (client specification).

The assessment can take place according to different methods(client-specific):

-   -   in five categories: satisfactory, slightly swollen, swollen,        coating damaged/coating detached or    -   on the basis of the temperature at which no visible change in        the coating surface subjected to stress can be discerned.

Examples of client specifications include the standard PA 15/050L of BMWand the standard PBODC 371 of Daimler Chrysler (Sindelfingen plant).

The Orange Peel Test (Waviness):

Wavy structures in the finished paint coat with a size of approximately0.1 to 10 nm are referred to as orange peel.

Such effects are often evaluated visually, i.e., subjectively aredescribed with terms such as “lumpy” or “grainy.” We see orange peel asa pattern of light and dark fields. The recognizability of thestructures depends on the observation distance.

-   -   Long waviness is discernible from a distance of approximately 3        m    -   Short waviness is visible only at a short distance        (approximately 50 cm)

To describe this effect in figures, the Wave-Scan plus from Byk-Gardneris used.

The surface (wavy brightness pattern) is scanned optically using a laserpoint light source at an angle of 60° and a detector on the oppositeside.

The measuring device is moved over a distance of 10 cm and the opticalbrightness profile is measured from point to point. The measurementsignal is divided into two components:

-   -   Long waviness (structures>0.6 mm)    -   Short waviness (structures<0.6 mm)

The values required by the automotive industry are:

-   -   Long waviness: 4-7 (very good)    -   Short waviness: 18-22 (very good)        Testing the Knoop Impression Hardness:

The hardness of an organic coating was determined by means of a smallload hardness tester (Leitz Miniload) from the company Leitz. Plasticdeformation of the coating is determined by measuring the length ofimpression caused by a tool (diamond tip) having a specified shape(rhomboid shape) and dimensions under defined test conditions (exposuretime, weight, temperature, etc.).

The length of impression is inversely proportional to the hardness ofthe coating film, i.e., the smaller the impression the harder is thecoating surface and the greater is the numerical value of the Knoophardness.

Further details (formulae for calculation, etc.) for the test conditionscan be found in the documents for the Knoop hardness tester from thecompany Leitz.

TABLE 1 Comparison Comparison Example 3-layer 4-layer Gloss 85 82 85Long waviness 5.4 7 5.4 Short waviness 21 23 21 Knoop hardness 23 23 23T-bend test 1.0 1.5 1.5 Adhesion OK OK complete separation Resistance tochemicals to tree resin (45° C.)*   57° C.   57° C.   57° C. todeionized water (>80° C.)* >80° C. >80° C. >80° C. to pancreatin (60°C.)*   70° C.   70° C.   70° C. to sulfuric acid 1% (56° C.)*   60° C.  60° C.   60° C. to sodium hydroxide solution >80° C. >80° C. >80° C.10% (>80° C.)* to fuel (okay)* OK OK OK *Values in parentheses arecurrent specifications from the automotive industry

Table I shows clearly that the clear coat layer according to theinvention meets all the requirements of the automotive industry and haseven better properties with regard to gloss and appearance in comparisonwith a three-layer coating, and adhesion is significantly improved incomparison with a four-layer coating with two identical unpigmented coatlayers.

1. Clear coat layer obtainable by (I) applying an unpigmentedintermediate coat to a substrate to be coated or to a color-impartingbase-coat layer, (II) crosslinking the intermediate coat and forming anintermediate coat layer, (III) applying an unpigmented top-coat to theintermediate coat layer, and (IV) crosslinking the top-coat and forminga top-coat layer, the intermediate coat layer having a greaterflexibility than the top-coat layer.
 2. Clear coat layer according toclaim 1, characterized in that step (1) is performed by applying anunpigmented intermediate coat to a color-imparting base-coat layer. 3.Clear coat layer according to claim 2, characterized in that thecolor-imparting basecoat layer is situated on a primer layer.
 4. Clearcoat layer according to claim 1, characterized in that the flexibilityof the intermediate coat layer is between T0 and T2, determinedaccording to the T-bend test.
 5. Clear coat layer according to claim 1,characterized in that the flexibility of the top-coat layer is betweenT0.5 and T5, determined according to the T-bend test.
 6. Clear coatlayer according to claim 1, characterized in that the value of theflexibility of the top-coat layer, determined according to the T-bendtest, is higher by 0.5 and 4 units, in particular by at least two units,than that of the intermediate coat layer.
 7. Clear coat layer accordingto claim 1, characterized in that the top-coat is obtainable bypolyaddition of a non-aqueous starting mixture comprising: (A) 10 wt. %to 70 wt. % of a non-aqueous solution of a polymer based on acrylatewith an OH number between 100 and 250, (B) 10 wt. % to 70 wt. % of anon-aqueous solution of a fluorine-modified polymer having a glasstransition temperature between 20 and 40° C., and (C) 20 wt. % to 60 wt.% of at least one blocked aliphatic or cycloaliphatic polyisocyanate,the weight ratio of component (A) to component (B) amounting to at most1, and the sum of the components (A), (B), and (C) amounting to 100%,based on the binder content of the starting mixture to be crosslinked.8. Clear coat layer according to claim 7, characterized in thatcomponent (A) is obtainable by radical polymerization of a monomermixture comprising: (i) 030 wt. % to 60 wt. % of at least onepolycycloaliphatic compound with at least two rings and a refractiveindex of at least 1.460 at 20° C., (ii) 25 wt. % to 70 wt. % of at leastone C₂-C₄ hydroxyallcyl (meth)acrylate with primary hydroxyl groups, and(iii) 0.1 to 1 wt. % acrylic acid, the sum of components (i), (ii), and(iii) amounting to 100 wt. %, based on the monomer mixture.
 9. Clearcoat layer according to claim 8, characterized in that the monomermixture additionally comprises 5 wt. % to 25 wt. % of a vinyl ester of abranched monocarboxylic acid having an average of 9 carbon atoms. 10.Clear coat layer according to claim 8, characterized in that thepolycycloaliphatic compound is isobornyl methacrylate.
 11. Clear coatlayer according to claim 8, characterized in that the polycycloaliphaticcompound of component (i) is selected from an acrylic copolymerobtainable by modifying an acrylic copolymer having at least one epoxygroup with a polycycloaliphatic substance comprising a carboxyl groupand having at least two rings with a refractive index of at least 1.460at 20° C., the epoxy group originating from glycidyl methacrylate. 12.Clear coat layer according to claim 11, characterized in that the molarratio of carboxyl group to epoxy group is between 0.5 and 1.0,preferably between 0.8 and 1.0, especially preferably between 0.9 and1.0.
 13. Clear coat layer according to claim 12, characterized in thatthe polycycloaliphatic substance comprising a carboxyl group comprises apolycycloaliphatic compound which has been additionally further reactedat elevated temperature with polycarboxylic acids and/or theiranhydrides to form a half-ester.
 14. Clear coat layer according to claim11, characterized in that the substance comprising a carboxyl group hasa refractive index of at least 1.480 at 20° C.
 15. Clear coat layeraccording to claim 11, characterized in that the polycy cloaliphaticsubstance comprising a carboxyl group is a tricycloaliphaticmonocarboxylic acid from the group of hydrogenated natural resin acids,adamantane carboxylic acids, and tricyclic monocarboxylic acids derivedfrom dicyclopentadiene, such as tricyclo[5.2.1.0.^(2,6)]decanee-8carboxylic acid, preferably tetrahydroabietic acid.
 16. Clear coat layeraccording to claim 11, characterized in that the polycy cloaliphaticsubstance comprising a carboxyl group is a reaction product of at leasttwo compounds, at least one of which is a polycycloaliphatic compoundhaving a refractive index of at least 1.460, preferably at least 1.480,at 20° C.
 17. Clear coat layer according to claim 16, characterized inthat at least one of the polycycloaliphatic compounds having arefractive index of at least 1.480 at 20° C. is comprised in an amountof at least 10 wt. %, preferably at least 20 wt. % and in particular atleast 50 wt. %, in the polycycloaliphatic reaction product comprising acarboxyl group.
 18. Clear coat layer according to claim 16,characterized in that the polycycloaliphatic compound is atricycloaliphatic monoalcohol from the group of perhydrogenated naturalresins, such as perhydroabietyl alcohol, the dicyclopentadienederivatives, such as 8-hydroxytricyclo[5.2.1.0.^(2,6)]decane,8-hydroxymethyltricyclo-[5.2.1.0.2,6]decane,8-hydroxytricyclo[5.2.1.0.^(2,6)]dec-3-ene,9-hydroxytricy-clo[5.2.1.0.^(2,6)]dec-3-ene.
 19. Clear coat layeraccording to claim 16, characterized in that the polycycloaliphaticcompound is a dicarboxylic acid and its anhydride from the group ofhydrogenated natural resin acids: adamantane carboxylic acids, andtricyclic monocarboxylic acids derived from dicyclopentadiene, e.g.tricyclo[5.2.1.0.^(2,6)]decane-3 carboxylic acid, preferablytetrahydroabietic acid.
 20. Clear coat layer according to claim 11,characterized in that the polycycloaliphatic substance comprising acarboxyl group additionally comprises one or more aromatic compounds.21. Clear coat layer according to claim 20, characterized in that thearomatic compound is selected from the group of aromatic monocarboxylicacids such as naphthoic acid; benzenemonocarboxylic acids such asbenzoic acid, o-toluic acid, m-toluic acid, ptoluic acid, hydroxybenzoicacid, tert-butylbenzoic acid, aromatic heterocyclic monocarboxylic acidssuch as pyridine carboxylic acids and furan carboxylic acids.
 22. Clearcoat layer according to claim 8, characterized in that theC₂-C₄hydroxyalkyl (meth)acrylate is selected from2-hydroxyethyl(meth)acrylateand 4-hydroxybutyl (meth)acrylate.
 23. Clearcoat layer according to claim 8, characterized in that up to 50% of theprimary hydroxyl groups of theC₂-C₄ hydroxyalkyl (meth)acrylate arereplaced by secondary hydroxyl groups.
 24. Clear coat layer according toclaim 23, characterized in that the C2 to C4 hydroxyalkyl (meth)acrylateis selected from 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl(meth)acrylate and hexanediol-1,6-mono(meth)acrylate.
 25. Clear coatlayer according to claim 7, characterized in that the fluorinemodifiedpolymer is a polymer based on fluorine-comprising vinyl ether with afluorine content between 25 and 30%, a glass transition temperaturebetween 16 and 45° C. and a hydroxyl value between 45 and
 90. 26. Clearcoat layer according to claim 7, characterized in that the blockedaliphatic or cycloaliphatic polyisocyanate is a blocked isophoronediisocyanate (IPDI,3,5,5-trimethyl-1-isocyanato-3-isocyanatomethylcyclohexane)and/or2,4,6-trioxo-1,3,5-tris(6-isocyanatohexyl)hexahydro-1,3,5-triazinepresent in trimerized form or in biuret form.
 27. Clear coat layeraccording to claim 1 characterized in that the intermediate coat isobtainable by crosslinking a non-aqueous starting mixture comprising:(A) 60 to 90 wt. % of a non-aqueous solution of at least onecycloaliphatic polyester with an OH number between 20 and 150 and aglass transition temperature between 0 and 70° C. and a weight-averagemolecular weight between 750 and 7000, and (B) 10 to 40 wt. % of atleast one blocked aliphatic or cycloaliphatic polyisocyanate, the sum ofcomponents (A) and (B) being 100%, based on the binder content of thestarting mixture.
 28. Clear coat layer according to claim 1characterized in that the thickness of the intermediate coat layer inthe crosslinked state is 10 to 25 μm.
 29. Clear coat layer according toclaim 1 characterized in that the thickness of the top-coat layer in thecrosslinked state is 10 to 25 μm.
 30. Clear coat layer according toclaim 1 characterized in that the intermediate coat layer additionallycomprises effect pigments, in particular aluminum particles.
 31. Clearcoat layer obtainable by (I) applying an unpigmented intermediate coatto a substrate to be coated, (II) crosslinking the intermediate coat andforming an intermediate coat layer, (III) applying an unpigmentedtop-coat to the intermediate coat layer, and (IV) crosslinking thetop-coat and forming a top-coat layer, the intermediate coat layerhaving a greater flexibility than the top-coat layer.
 32. A method ofpreparing a clear coat layer comprising: (I) applying an unpigmentedintermediate coat to a substrate to be coated or to a color-impartingbase-coat layer; (II) crosslinking the intermediate coat and forming anintermediate coat layer, (III) applying an unpigmented top-coat to theintermediate coat layer, and (IV) crosslinking the top-coat and forminga top-coat layer, the intermediate coat layer having a greaterflexibility than the top-coat layer.
 33. A method of preparing amultilayer coating comprising: (1) applying a primer layer to apretreated metallic substrate and baking the same at a temperaturebetween 180 and 260° C.; (2) applying a color-imparting base-coat layerand baking the same at a temperature between 180 and 260° C.; and (3)applying a clear coat layer by applying an unpigmented intermediate coatto the substrate to be coated, crosslinking the intermediate coat andforming an intermediate coat layer, applying an unpigmented top-coat tothe intermediate coat layer, and crosslinking the top-coat and forming atop-coat layer; the intermediate coat layer having a greater flexibilitythan the top-coat layer.